载荷不确定的周期性结构稳健拓扑优化

提出了一种考虑周期性几何约束的结构稳健拓扑优化设计方法。针对线弹性体结构,提出了载荷不确定性条件下周期性结构稳健拓扑优化模型;推导了周期性结构柔度均值与方差的敏度数表达式,并基于软删双向渐进结构优化法提出了载荷不确定性条件下的周期性结构稳健拓扑优化设计方法。2个不同约束条件下的实例表明:本文提出的优化模型稳定性较好;考虑载荷不确定性得到的结构与确定性载荷下得到的结构有很大的区别,且稳健性优化设计得到的结构更加稳定。     

Aerodynamic periodicity of transient aerodynamic forces of flexible plunging airfoils

For flapping micro-air vehicles, periodical aerodynamics in every plunging period is always desired in the design of aircraft controlling system. During the periodic plunges of rigid airfoil, either periodical or non-periodical aerodynamic forces can be generated. Real airfoils are usually flexible so that the effect of flexibility on unsteady characteristics of aerodynamic periodicity should be considered. In this study, a fluid structure interaction analysis is employed to systematically investigate the aerodynamic periodicity of a periodically plunging flexible airfoil. The influences of several dimensionless kinematic and structural parameters on aerodynamic periodicity are investigated. The results show that the aerodynamic periodicity of plunging flexible airfoil qualitatively resembles that of rigid airfoil as the mechanisms contributing to aerodynamic periodicity are kept for airfoil with various flexibilities. However, aerodynamic periodicity at small plunging amplitude and high Reynolds number can be improved as airfoil becomes medium flexible but worsened as airfoil is very flexible. The reason is that the deformation of the airfoil changes the strength and structure of the nearby vortices, thereby leading to weak vortex–vortex and vortex–airfoil interactions. The extent of improvement in aerodynamic periodicity is also found significantly influenced by incoming velocity.     

Quasi-periodicities in cosmic rays recorded by the KACST muon detector during 2002–2012

In this study, daily cosmic ray data obtained with the KACST muon detector for the period 2002–2012 were analyzed for quasi-periodicities. Power-spectrum analysis was carried out and several periodicities were identified. The results reveal several periodicities at different frequency scales: 817 days (~2.19 years), 617 days (~1.7 years), 475 days (~1.3 years), 421 days (1.15 years), 290 days (~0.8 years), 227 days (~0.62 years), 185 days (~0.52 years), 153 days, 135 days, 120 days, 93 days, 84 days, 73 days, 65 days, 53–45 days, 38 days, 31 days, 25–27 days, 21 days and 13 days. The obtained periodicities are in an agreement with those previously reported by several investigators.The identified periodicities have strong relevance to solar activity parameters such as variations in the interplanetary magnetic field.In comparison, the data from two neutron monitors (NMs), Lomnický ?tít and Oulu NMs, for the same period were used and their power spectra were correspondingly obtained. Similarities and differences in the position of the cosmic-ray peaks measured by the KACST muon detector and by the two NMs have been presented and discussed. While most of the periodicities reported by the muon detector are also found in the NM data, the ~1.7-yr variation is not found. Instead, a shift of 1.6–1.8 years in the NM data is observed which may be due to the limited epoch considered in this study.     

膜层厚比对CrN_x/Ti_yCr_(1-y)N多层膜硬度与结合力影响研究

为研究膜层交替周期对多层膜表面硬度与耐磨性能的影响,利用金属蒸发真空弧离子源(MEVVA)和磁过滤阴极真空弧复合离子束沉积技术在AM355钢材表面制备膜层交替周期分别为5、13和26的3组CrN_x/Ti_yCr_(1-y)N膜层,利用场发射扫描电子显微镜(FESEM)和X射线衍射仪(XRD)对膜层形貌和相结构进行了分析,利用划痕仪、显微硬度计等对膜层硬度与结合力进行了测试。结果表明:采用该技术制备的CrN_x/Ti_yCr_(1-y)N多层膜结构致密,与基体结合良好;随着多层膜交替周期的增加,膜层硬度有所降低,膜层结合力则呈现先升高后缓慢降低的趋势。     

一种鲁棒的弹道目标RCS周期估计方法

由于运动方式、姿态控制等方面的差异,弹道中段目标(弹头、诱饵)存在不同的RCS周期特性,这是目标识别的重要特征.分析了弹头目标的RCS周期特性,指出了传统频域分析方法的不足,提出了一种新的RCS周期估计方法:通过利用循环平均幅度差函数,有效克服了传统频域方法的不足,且算法具有优良的抗噪性能.实验结果表明该方法能明显改善RCS周期估计的精确性和稳定性.